Over the past several decades, highly sophisticated techniques have been developed for identifying and producing hydrocarbons, commonly referred to as oil and gas, from subsurface formations. These techniques facilitate the discovery, assessment, and production of hydrocarbons from subsurface formations.
Oil and gas producers typically image subterranean rock layers to determine the location and shape of the subterranean rock layers. Detection and characterization of permeable fractures are of great interest in oil exploration because they are related to the transportation and storage of hydrocarbons and, therefore, provide information that is useful to optimize production of the hydrocarbons. A resistivity tool is widely used to detect such fracture by imaging cracks on a borehole wall. Alternatively, a sonic tool can be used to find Stoneley wave reflections and transmissions that imply the existence of such open fractures. However, the foregoing types of tools and measurements are limited to vicinity of the borehole wall and, therefore, are not useful to estimate the extent of a fracture. To determine the extent of a fracture, measurements using waves that penetrate into the formation are necessary. Further, while the event signals from fractures can be evident in sonic waveforms, Borehole Acoustic Reflection Survey (BARS) (a sonic imaging service available from Schlumberger) is rarely used to detect fractures because the acoustic and elastic responses from a fracture are not well understood.